Modern turbines often operate at extremely high temperatures. The effect of temperature on the turbine blades and/or stator vanes can be detrimental to the efficient operation of the turbine and can, in extreme circumstances, lead to distortion and possible failure of the blade or vane. In order to overcome this risk, high temperature turbines may include hollow blades or vanes incorporating so-called impingement tubes for cooling purposes.
These so-called impingement tubes are hollow tubes that run radially within the blades or vanes. Air is forced into and along these tubes and emerges through suitable apertures into a void between the tubes and interior surfaces of the hollow blades or vanes. This creates an internal air flow for cooling the blade or vane.
Normally, blades and vanes are made as precision castings having hollow structures in which impingement tubes are inserted for impingement cooling of an impingement cooling zone of the hollow structure. Problems arise when a cooling concept is used in which a temperature of a cooling medium for the impingement cooling zone is too high for efficient cooling of the latter.
This is known from a cooling concept, where a combined platform and aerofoil cooling systems are arranged in series. A compressor discharge flow feeds in the platform cooling and then passes into the aerofoil cooling system. All the cooling flow is discharged through the aerofoil. In the absence of film cooling, all the flow can be discharged through the aerofoil trailing edge.
The technical problem relates to the combined platform and aerofoil cooling system. One of the main disadvantages with such a system is the elevated cooling air temperatures supplied to the aerofoil section, resulting from the heat pickup of the platform cooling. The increase in cooling air temperature can be of the order of 50° C. When engines are significantly up-rated, the resultant coolant temperature rise through the platform cooling can be a significant factor limiting ability to achieve the required cooling levels within the aerofoil. In such situations a significant redesign of the cooling or change of cooling feed system may be required, involving a significant amount of development and production time and cost. A change of cooling feed system to an state of the art independent aerofoil/platform system can have the disadvantage of increased aerodynamic/performance losses, since more cooling air is discharged in the gas path in a less efficient manner, i.e. near the platform regions at undesired trajectories.